1. Field of the Invention
The present invention relates to dielectric filters for use in the microwave band, to dielectric duplexers, and to communication apparatuses including the same.
2. Description of the Related Art
As known filters for use in the microwave band, dielectric filters formed by a single-stage or multi-stage resonators including a dielectric block containing therein inner-conductor-formed holes and an outer conductor formed on the exterior surface of the dielectric block have been used.
The dielectric filter using the dielectric block includes, on the exterior surface of the dielectric block, input/output terminals which are capacitively coupled to inner conductors, and hence signals are input and output in an unbalanced manner. In order to supply a signal to a balanced-input amplifier circuit, a balun (unbalanced-to-balanced transformer) is used to transform an unbalanced signal into a balanced signal. With this arrangement, the balun has a high insertion loss. It is necessary to have enough space for disposing the balun on a circuit board, and hence the dielectric filter cannot be miniaturized.
The assignee of the present invention has submitted Japanese Patent Application No. 11-314657 and Japanese Patent Application No. 2000-036302 relating to a dielectric filter which is a balanced filter for inputting and outputting signals.
In a dielectric filter which is a balanced filter for inputting and outputting signals, the ideal phase difference between balanced input/output terminals is 180 degrees, and the ideal amplitude difference is zero.
In the dielectric filter with the balanced input/output terminals, filter characteristics differing from those obtained by resonance in a TEM mode by the dielectric block and the inner and outer conductors included therein may be generated. When filter characteristics differing from those expected from the design are generated, the ideal relationship, that is the phase difference between the balanced input/output terminals being 180 degrees and the amplitude difference being zero, cannot be achieved over a wide frequency band.
It can be estimated from various experimental results obtained by the inventors of the present invention that a spurious mode, such as a TE mode, occurs due to the dielectric block and the outer conductor on the exterior surface of the dielectric block. The resonant frequency in the spurious mode influences the operating frequency band, and it can be considered that this influence causes deterioration of balance characteristics.
Accordingly, it is an object of the present invention to provide a dielectric filter for maintaining balance characteristics over a wide frequency band, a dielectric duplexer, and a communication apparatus including the same.
In accordance with an aspect of the present invention, a dielectric filter is provided including a dielectric block including a plurality of inner-conductor-formed holes extending from a first face of the dielectric block to a second face opposed to the first face; inner conductors formed inside the inner-conductor-formed holes, portions in the vicinity of both ends of the inner conductors being open; balanced input/output terminals formed on the exterior surface of the dielectric block, the balanced input/output terminals being capacitively coupled to portions in the vicinity of open ends of the inner conductor in a predetermined inner-conductor-formed hole of the plurality of inner-conductor-formed holes; an unbalanced input/output terminal formed on the exterior surface of the dielectric block, the unbalanced input/output terminal being capacitively coupled to a portion in the vicinity of one open end of the inner conductor in one of the other inner-conductor-formed holes; an outer conductor formed on the exterior surface of the dielectric block; and a ground electrode connected to the outer conductor, the ground electrode being formed on one aperture face of the dielectric block, the aperture face having apertures of the inner-conductor-formed holes and the unbalanced input/output terminal, or the ground electrode being formed at a predetermined distance from the aperture face. Thus, the dielectric filter can have balanced input/output without being influenced by a spurious mode, such as a TE mode.
In accordance with another aspect of the present invention, a dielectric filter is provided including a dielectric block including a plurality of inner-conductor-formed holes extending from a first face of the dielectric block to a second face opposed to the first face; inner conductors formed inside the inner-conductor-formed holes, portions in the vicinity of both ends of the inner conductors being open; first balanced input/output terminals formed on the exterior surface of the dielectric block, the first balanced input/output terminals being capacitively coupled to portions in the vicinity of open ends of the inner conductor in a predetermined inner-conductor-formed hole of the plurality of inner-conductor-formed holes; second balanced input/output terminals formed on the exterior surface of the dielectric block, the second balanced input/output terminals being capacitively coupled to portions in the vicinity of open ends of the inner conductor in one of the other inner-conductor-formed holes; an outer conductor formed on the exterior surface of the dielectric block; and a ground electrode connected to the outer conductor, the ground electrode being formed on one aperture face of the dielectric block, the aperture face having apertures of the inner-conductor-formed holes, or the ground electrode being formed at a predetermined distance from the aperture face. Thus, the dielectric filter can have balanced input/output without being influenced by a spurious mode, such as a TE mode.
With this arrangement, one of the aperture faces having the apertures of the inner-conductor-formed holes can function as a short-circuit conductor in a resonant mode such as a TE mode due to the dielectric block and the outer conductor. Thus, the resonant frequency in a spurious mode such as a TE mode can be widely shifted, and the influence of the spurious mode can be avoided.
The ground electrode may include a metal cover for covering a portion in the vicinity of the aperture face having the apertures of the inner-conductor-formed holes.
Thus, the influence by a spurious mode, such as a TE mode, can be easily avoided without changing the dielectric block.
The ground electrode may include an electrode film formed on a protrusion protruding from the aperture face of the dielectric block, the aperture face having the apertures of the inner-conductor-formed holes, or formed in a recess bored in the aperture face. Thus, the influence of a spurious mode, such as a TE mode, can be easily avoided without externally providing a metal cover.
The ground electrode may include an electrode film formed on one aperture face of the dielectric block, the aperture face having the apertures of the inner-conductor-formed holes. Thus, the influence of a spurious mode, such as a TE mode, can be easily avoided without externally providing a metal cover.
In accordance with another aspect of the present invention, a dielectric duplexer is provided including a dielectric filter with any one of the foregoing structures. Thus, for example, attenuation in an adjacent frequency band between a transmission filter and a reception filter can be increased. For example, balanced input/output can be performed while a transmission signal is reliably prevented from entering the reception filter.
In accordance with yet another aspect of the present invention, a communication apparatus including the foregoing dielectric filter or the foregoing dielectric duplexer is provided. Thus, a small communication apparatus with highly efficient communication characteristics can be provided without using a balanced-unbalanced transformer.